Interpretive Summary: The importance of the spleen in the immunity of cattle to blood parasites like Babesia bovis is well known, but the precise mechanisms involved are not fully understood. Recent studies have identified a number of cell populations within the spleen that work in concert to effect the demise of the invading pathogen. In this study, we demonstrated that cells first encountering foreign pathogens can interact with other cells to produce products detrimental to the pathogen. Therefore, vaccines developed in the future must be capable of activating these cell populations.

Technical Abstract:
Early interactions of innate immune cell populations such as DC, monocytes/macrophages and NK cells, can affect the ability of the acquired immune response to control infection of intracellular microorganisms. In this study, we investigated the activation of bovine NK cells by CD13+ splenic DC or CD172a+ blood monocytes stimulated with either Mycobacterium bovis BCG or Babesia bovis merozoites. Splenic DC exposed to GM-CSF, IL-4 and Flt3L (cytokine+) and then stimulated with B. bovis merozoites induced significantly more IFN-gamma production from NK cells that cytokine-unexposed (cytokine-) cells, suggesting that the stage of DC maturation is important for NK cell activation. In contrast, cytokine- monocytes stimulated with M. bovis BCG induced significantly more NK cell IFN-gamma production than cytokine+ monocytes. Interestingly, cytotoxicity and perforin up-regulation were more pronounced in NK cells cultured with with cytokine - rather than cytokine+ DC or monocytes. The data provide evidence that both DC and monocytes can act as regulatory and effector accessory cells inducing IFN-gamma production and cytotoxic activity by NK cells. However, there were significant differences related to accessory cell phenotype, maturation state and type of microbial stimulation. Taken together, this study demonstrates the activation of bovine NK cells by microbial-stimulated splenic DC or blood monocytes, which may have an important role in controlling intracellular infections.